1. Field of the Invention
The present invention relates to a polyimide precursor and a cured product thereof which have a small dielectric constant, a small thermal expansion coefficient and a high heat resistance, and processes for producing them.
2. Prior Art
Polyimides have heretofore been known as resins having a high heat resistance. In general, they are obtained, for example, by a process which comprises subjecting a diamine component and a tetracarboxylic acid dianhydride component to polymerization in an organic solvent to form a poly(amic acid), followed by dehydrating-ring closure of the poly(amic acid).
As examples of the polyimides, there are known (a) novel polyimides comprising structural units represented by the general formula (I) or the general formula (II): ##STR4## (wherein R' is a divalent hydrocarbon group), and poly(amic acid)s or poly(amic acid) esters which are precursors of the polyimides (Japanese Patent Application Kokai Nos. 62-265327 and 63-10629):
There are also known (b) polyimides comprising repeating units represented by the general formula (III): ##STR5## wherein R" is a tetravalent aliphatic or aromatic group, and n is 1 or 2 (Japanese Patent Application Kokai Nos. 57-114258, 57-188853, 60-250031 and 60-221426).
There are also known (c) polyimides comprising repeating units represented by the general formula (IV): ##STR6## wherein Y is --C(CH.sub.3).sub.3 --, --C(CF.sub.3).sub.3 --or --SO.sub.2 --(Japanese Patent Application Kokai Nos. 62-231935, 62-231936 and 62-231937).
There are also known (d) polyimides having a small dielectric constant which are obtained from 2,2-bis-(3,4-dicarboxyphenyl)propanoic dianhydride or 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanoic dianhydride and an aromatic diamine such as 4,4'-bis(4-aminophenoxy)biphenyl of 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (Japanese Patent Application Kokai 2-60934).
There are also known (e) polyimides obtained from 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2-bis-(3-aminophenyl)hexafluoropropane and a mixed acid dianhydride consisting of pyromellitic dianhydride and an acid dianhydride having a diaryl nucleus (Japanese Patent Application Kokai 2-67320), and polyimides obtained from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanoic dianhydride, 2,2-bis(4-aminophenyl)hexafluoropropane and 2,2-bis(3-aminophenyl)hexafluoropropane (Japanese Patent Application Kokai 2-86624).
The above-mentioned polyimides (a), (b), (c), (d) and (e) do not satisfy all of the requirements such as high heat resistance, small dielectric constant, small thermal expansion coefficient, high mechanical properties (in particular, high flexibility) and high glass transition temperature. The polyamides (a), (b) and (c) are excellent with respect to high heat resistance, small thermal expansion coefficient and high glass transition temperature, but have a large dielectric constant and a low flexibility. It can be speculated that the reason for their advantages such as high heat resistance, high glass transition temperature and small thermal expansion coefficient and their defects such as large dielectric constant and low flexibility is that these polymers have a relatively large number of imide rings and comprise structural units represented by the formula ##STR7## The polyamides (d) and (e) have a small dielectric constant and a high flexibility because they contain --CF.sub.3 (trifluoromethyl groups) and --O--linkages. But, they have a low heat resistance, a low glass transition temperature and a large thermal expansion coefficient, and moreover it is conjectured that because of their --CF.sub.3 groups, they have a low resistance to an alkali solution, for example, as electroless plating solution.